It is especially forbidden to employ arms, projectiles, or material calculated to cause unnecessary suffering.

What weapons cause “unnecessary injury” can only be determined in light of the practice of States in refraining from the use of a given weapon because it is believed to have that effect … Usage, has, however, established the illegality of the use of lances with barbed heads, irregular-shaped bullets, and projectiles filled with glass, the use of any substance on bullets that would tend unnecessarily to inflame a wound inflicted by them, and the scoring of the surface or the filing off of the ends of the hard cases of bullets.

United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 34.

The US Air Force Pamphlet (1976) provides:

It is forbidden to employ weapons, projectiles, and material and methods of warfare of a nature to cause superfluous injury or unnecessary suffering. This rule is a matter of customary international law …

The rule prohibiting the use of weapons causing unnecessary suffering or superfluous injury is firmly established in international law … This prohibition against unnecessary suffering is a concrete expression of the general principles of proportionality and humanity. The rule reflects interests of combatants in avoiding needless suffering. Weapons are lawful, within the meaning of the prohibition against unnecessary suffering, so long as the foreseeable injury and suffering associated with wounds caused by such weapons are not disproportionate to the necessary military use of the weapon in terms of factors such as effectiveness against particular targets and available alternative weapons. What weapons or methods of warfare cause unnecessary suffering, and hence are unlawful per se, is best determined in the light of the practice of states. All weapons cause suffering. The critical factor in the prohibition against unnecessary suffering is whether the suffering is needless or disproportionate to the military advantages secured by the weapon, not the degree of suffering itself. International agreements may give specific content to the principle in the form of specific agreements to refrain from the use of particular weapons or methods of warfare. Thus, international law has condemned dum dum or exploding bullets because of types of injuries and inevitability of death.

United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 6-3(b)(1) and (2).

[emphasis in original]

The Pamphlet also states: “The long-standing customary prohibition against poison is based on their uncontrolled character and the inevitability of death or permanent disability.”

United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 6-4f.

The Pamphlet adds:

A new weapon or method of warfare may be illegal, per se, if it is restricted by international law including treaty or international custom … [T]he legality of new weapons … is determined by whether the weapon’s effects violate the rule against unnecessary suffering.

United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, 6-7(a).

The US Air Force Commander’s Handbook (1980) states:

Weapons that cause unnecessary suffering or superfluous injury are prohibited. Note that the degree of suffering is not the principal issue; the true test is whether the suffering is needless or disproportionate to the military advantage expected from the use of the weapon.

(1) Thus, poisoned bullets are felt to cause unnecessary suffering since a person injured by modern military ammunition will ordinarily be placed out of the fighting by that alone; there is very little military advantage to be gained [by] making sure of the death of wounded persons through poison since they will be out of the battle when the poison takes effect.

(2) Similarly, using clear glass as the injuring mechanism in an explosive projectile or bomb is prohibited, since glass is difficult for surgeons to detect in a wound and impedes treatment.

United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 6-2a.

The US Soldier’s Manual (1984) states: “The law of war does not allow you to alter your weapons in order to cause unnecessary injury or suffering to the enemy.”

United States, Your Conduct in Combat under the Law of War, Publication No. FM 27-2, Headquarters Department of the Army, Washington, November 1984, p. 11; see also p. 10.

The US Instructor’s Guide (1985) states:

The customary law of war and the [1907] Hague Regulations … limit the weapons the armed force can use. Under the Hague Regulations, the employment of arms, material, or projectiles designed to cause unnecessary suffering is prohibited. These principles have outlawed irregular-shaped bullets such as dum-dum bullets, projectiles filled with glass, and any substances or projectiles that would tend to inflame a wound. [The US] Field Manual 27-10 states, in paragraph 34, that whether weapons cause unnecessary injury “… can only be determined in the light of the practice of the States in refraining from the use of a given weapon because it is believed to have that effect” …

It is possible … for a soldier to violate the law of war by misusing an issued weapon or using it at the wrong time or in the wrong place. An example of misusing a legitimate weapon would be cutting off the tip of a bullet. When the bullet hits someone, it expands and leaves a gaping wound. Such bullets cause unnecessary suffering and are forbidden. This misuse of a legitimate weapon is a crime for which you can be prosecuted.

United States, Instructor’s Guide – The Law of War, Headquarters Department of the Army, Washington, April 1985, p. 7.

The US Operational Law Handbook (1993) states that “using weapons which cause unnecessary suffering” is “expressly prohibited by the law of war and [is] not excusable on the basis of military necessity”.

United States, Operational Law Handbook, JA 422, Center for Law and Military Operations and International Law Division, The Judge Advocate General’s School, United States Army, Charlottesville, Virginia 22903-1781, 1993, p. Q-182, § (i).

The US Naval Handbook (1995) provides:

It is a fundamental tenet of the law of armed conflict that the right of nations engaged in armed conflict to choose methods or means of warfare is not unlimited. This rule of law is expressed in the concept that the employment of weapons, material, and methods of warfare that are designed to cause superfluous injury or unnecessary suffering is prohibited.

…

Antipersonnel weapons are designed to kill or disable enemy combatants and are lawful notwithstanding the death, pain, and suffering they inflict. Weapons that are designed to cause unnecessary suffering or superfluous injury are, however, prohibited because the degree of pain or injury, or the certainty of death they produce is needlessly or clearly disproportionate to the military advantage to be gained by their use. Poisoned projectiles and small arms ammunition intended to cause superfluous injury or unnecessary suffering fall into this category. Similarly, using materials that are difficult to detect or undetectable by field x-ray equipment, such as glass or clear plastic, as the injuring mechanism in military ammunition is prohibited, since they unnecessarily inhibit the treatment of wounds.

United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), §§ 9.1 and 9.1.1.

The US Naval Handbook (2007) states:

The law of armed conflict prohibits the use of arms, projectiles, or material calculated to cause unnecessary suffering to combatants. … [C]ommanders should ensure that otherwise lawful weapons or munitions are not being altered or misused to cause greater or unnecessary suffering.

United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 5.3.4.

The Handbook also states:

It is a fundamental tenet of the law of armed conflict that the right of nations engaged in armed conflict to choose methods or means of warfare is not unlimited. This rule of law is expressed in the concept that the employment of weapons, material, and methods of warfare that are designed to cause superfluous injury or unnecessary suffering is prohibited.

United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 9.1.

The Handbook further states:

Antipersonnel weapons are designed to kill or disable enemy combatants and are lawful notwithstanding the death, pain, and suffering they inflict. Weapons that are designed to cause unnecessary suffering or superfluous injury are, however, prohibited because the degree of pain or injury, or the certainty of death they produce is needlessly or clearly disproportionate to the military advantage to be gained by their use. Poisoned projectiles and small arms ammunition intended to cause superfluous injury or unnecessary suffering fall into this category. Similarly, using materials that are difficult to detect or undetectable by field x-ray equipment, such as glass or clear plastic, as the injuring mechanism in military ammunition is prohibited, since they unnecessarily inhibit the treatment of wounds. Use of such materials as incidental components in ammunition, e.g., as wadding or packing, is not prohibited. Use of .50-caliber weapons against individual enemy combatants does not constitute a violation of this proscription against unnecessary suffering or superfluous injury.

United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 9.1.1.

The Agent Orange case in 2005 involved a class action suit filed on behalf of various Vietnamese nationals and an organization, The Vietnamese Association for Victims of Agent Orange/Dioxin, against Dow Chemical and other US chemical manufacturers, for harms allegedly done to them and their land through the United States’ use of Agent Orange and other herbicides during the Vietnam War from 1965 to 1971 and by the South Vietnamese Government’s subsequent use of such herbicides until 1975. In dismissing the claims, the Court found that, while recognizing the evolution of international law since 1975, the use of herbicides did not violate, at the time they were used, either customary or conventional international law binding on the United States. On the question of whether the use of herbicides is calculated to cause unnecessary suffering, the Court stated:

Plaintiffs argue that herbicides are per se proscribed materials that are “calculated to cause unnecessary suffering.” See Hague Convention IV, art. 23(e), 36 Stat. at 2302. The prohibition has long been understood to apply to weapons that cause unnecessary or superfluous injury to an already incapacitated combatant. Anderson Decl [Decl. of Professor Kenneth Howard Anderson, Jr., Nov. 2, 2004] ¶ 47 (citing, as an example, bullets treated with an agent that inflames a wound). It does not apply to lethal weapons that collaterally cause injury or suffering in order to achieve an appropriate (in war) military objective of incapacitating enemy troops, let alone to herbicides that may cause harm to humans beings as the side-effect of an application intended to destroy plants. As a leading treatise explains, Article 23’s prohibition of weapons “calculated to cause unnecessary suffering” is:

too vague to produce by itself a great many practical results. Apart from cases in which states expressly agree to forbid employment of a specified weapon …, states have not been known to lightly decide unilaterally to discard a weapon, once introduced into their arsenals, because it is considered to cause unnecessary suffering.

FRITS KALSHOVEN & LIESBETH ZEGVELD, CONSTRAINTS ON THE WAGING OF WAR: AN INTRODUCTION TO INTERNATIONAL HUMANITARIAN LAW 41–42 (3d ed. 2001) …

Clause (e) is not applicable because the herbicides were not “calculated to cause unnecessary suffering.” The herbicide spraying was in the course of armed activities deemed necessary by the United States to protect American armed forces and those of its allies. It was not designed to harm people or land independently as a punishment or to inflict hurt viciously and consciously.

In 1974, in reply to a letter from a member of the US House of Representatives, the Acting General Counsel of the US Department of Defense stated:

The distinguishing feature in Article 23 of the [1907 Hague Regulations] is that it applies to all weapons, and qualifies the use of all weapons in armed conflict making unlawful uses which cause suffering intentionally superfluous to a valid military purpose. The term “unnecessary suffering” conveys this interpretation. The terms “calculated to cause” convey the element of intent such that members of the Armed Forces cannot justify the use of weapons inconsistent with attaining a legitimate military objective. This criterion must be distinguished from prohibitions agreed to by states for outlawing weapons regardless of how they are used or intended to be used. As noted in the Field Manual … one must refer to the practices of states in order to determine the present meaning of these principles.

United States, Reply of 18 January 1974 of the Acting General Counsel of the Department of Defense to a letter of a member of the House of Representatives, reprinted in Arthur W. Rovine, Digest of United States Practice in International Law, 1974, Department of State Publication 8809, Washington, D.C., 1975, pp. 705–706.

At the Conference of Government Experts on Weapons which may Cause Unnecessary Suffering or have Indiscriminate Effects, held in Lucerne in 1974, the United States stated:

The prohibition against weapons that cause unnecessary suffering is a criterion to which we are currently bound under the Fourth Hague Convention of 1907, but interpretations of its scope and implications today vary significantly. It is the U.S. view that the “necessity” of the suffering must be judged in relation to the military utility of the weapons. The test is whether the suffering is needless, superfluous, or disproportionate to the military advantage reasonably expected from the use of the weapon.

United States, Statement of 25 September 1974 at the Conference of Government Experts on Weapons which may Cause Unnecessary Suffering or have Indiscriminate Effects, Lucerne, 24 September-18 October 1974, reprinted in Arthur W. Rovine, Digest of United States Practice in International Law, 1974, US Department of State Publication 8809, Washington, D.C., 1975, p. 707.

In 1979, during the Preparatory Conference to the UN Conference that led to the adoption of the 1980 Convention on Certain Conventional Weapons, the United States stated, in a discussion on incendiary weapons, that “some delegations had based themselves on the premise that incendiary weapons caused unnecessary suffering and were, by definition, inhumane, but if that premise was correct, they would already have been outlawed”.

United States, Statement at the Preparatory Conference to the UN Conference on prohibitions or restrictions of certain conventional weapons which may be deemed to be excessively injurious or to have indiscriminate effects, 12 April 1979, UN Doc. A/CONF.95/PREP.CONF./II/SR.28, 18 April 1979, pp. 2–3.

Upon ratification of the 1980 Convention on Certain Conventional Weapons, the United States declared:

With reference to the scope of application defined in article 1 of the Convention, … the United States will apply the provisions of the Convention, Protocol I, and Protocol II to all armed conflicts referred to in articles 2 and 3 common to the Geneva Conventions for the Protection of War Victims of August 12, 1949.

United States, Declaration made upon ratification of the Convention on Certain Conventional Weapons, 24 March 1995.

In 1987, the Deputy Legal Adviser of the US Department of State affirmed:

We support the principle that the permissible means of injuring the enemy are not unlimited and that parties to a conflict not use weapons, projectiles, and materials and methods of warfare of a nature to cause superfluous injury or unnecessary suffering.

United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 424.

In 1988, in a memorandum on laser weapons, the US Department of the Army affirmed:

Article 23(e) [of the 1907 Hague Regulations] prohibits the employment of “arms, projectiles, or material calculated to cause unnecessary suffering.” There is no internationally accepted definition of “unnecessary suffering.” In fact, an anomaly exists in that while it is legally permissible to kill an enemy soldier, in theory any wounding should not be calculated or intended to cause unnecessary suffering. In endeavoring to reconcile the two, in considering the customary practice of nations during this century, and in acknowledging the lethality of the battlefield for more than a century, certain factors emerge that are germane to this opinion:

(a) No legal obligation exists or can exist to limit wounding mechanisms in a way that permits lawful killing while requiring that wounds merely temporarily disable, that is, that the effects of wounds do not extend beyond the period of hostilities; and

(b) In considering whether a weapon may cause unnecessary suffering, it must be viewed in light of comparable wounding mechanisms extant on the modern battlefield rather than viewing the weapon in isolation.

(c) The term “unnecessary suffering” implies that there is such a thing as “necessary suffering,” i.e., that ordinary use of any military effective weapon will result in suffering on the part of those against whom it is employed.

(d) The rule does prohibit deliberate design or alteration of a weapon solely for the purpose of increasing the suffering of those against whom it is used, including acts that will make their wounds more difficult to treat. This is the basis for rules against poisoned weapons and certain small caliber hollow point ammunition.

United States, Department of the Army, Office of the Judge Advocate General, Memorandum of law concerning the legality of the use of lasers as antipersonnel weapons, 29 September 1988, § 4, reprinted in Marian Nash, Cumulative Digest of United States Practice in International Law, 1981–1988, Department of State Publication 10120, Washington, D.C., 1993–1995, p. 3424.

Course material from the US Army War College, in discussing the balance between military necessity and unnecessary suffering, states that the “existence of a weapon generally indicates a legitimate military requirement” and maintains that no effective weapon has ever been outlawed. The example used to prove this statement is the 1907 Hague Convention (VIII), which bans anchored automatic contact mines which do not become harmless when they break loose from their mooring and torpedoes that do not become harmless after they have missed their target. In contrast to this, the course material points to the 1899 Hague Declaration concerning Asphyxiating Gases, which the United Kingdom and the United States did not ratify, and to the fact that poison gas was used during the First World War.

In 1990, in a memorandum of law on sniper use of open-tip ammunition, the US Department of the Army stated:

Although the United States has made the formal decision that for military, political, and humanitarian reasons it will not become a party to [the 1977 Additional Protocol I], U.S. officials have taken the position that the language of article 35(2) of Protocol I … is a codification of customary international law, and therefore binding upon all nations.

United States, Department of the Army, Office of the Judge Advocate General, Memorandum of Law on Sniper Use of Open-Tip Ammunition, 12 October 1990, § 3.

In 1992, in a review of the legality of extended range anti-armour munition, the US Department of the Air Force stated: “International law prohibits the use, even against military objectives, of weapons which cause unnecessary suffering or superfluous injury.”

In 1993, in a legal review of the USSOCOM Special Operations Offensive Handgun, the Judge Advocate General of the US Department of the Army stated:

Although President Ronald Reagan declined to submit [the 1977 Additional Protocol I] to the Senate for its advice and consent to ratification, the U.S. Government considers the language quoted from article 35(2) of Protocol I to be a codification of customary international law to the extent that it prohibits superfluous injury, as prohibited by Article 23e of the … [1907 Hague Regulations], and therefore binding upon all nations.

United States, Department of the Army, Office of the Judge Advocate General, Legal Review of USSOCOM Special Operations Offensive Handgun, 16 February 1993, p. 11.

In its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, the United States stated that it “has long taken the position that various principles of the international law of armed conflict would apply to the use of nuclear weapons as well as to other means and methods of warfare”. It added that the prohibition on the use of weapons of a nature to cause superfluous injury or unnecessary suffering “was intended to preclude weapons designed to increase the injury or suffering of the persons attacked beyond that necessary to accomplish the military objective. It does not prohibit weapons that may cause great injury or suffering if the use of the weapon is necessary to accomplish the military mission.”

United States, Written statement before the ICJ, Nuclear Weapons case, 20 June 1995, p. 21.

In its oral pleadings before the ICJ in the Nuclear Weapons case in 1995, the United States stressed:

Returning to the claims that have been made regarding specific principles of the law of armed conflict, it has also been argued that nuclear weapons categorically cause unnecessary suffering or superfluous injury and therefore violate the law of armed conflict. This line of argument cannot be sustained. The unnecessary suffering principle prohibits the use of weapons designed specifically to increase the suffering of persons attacked beyond that necessary to accomplish a particular military objective. As a general matter, however, it does not prohibit the use of weapons that cause great injury and pain, as such. Under this principle, whether use of a particular weapon causes unnecessary suffering depends, therefore, on whether its use and resultant effects are required to accomplish a legitimate military objective, a question which again cannot be answered in the abstract.

In 1997, in a message to the US Senate analysing Article 3(3) of the 1996 Amended Protocol II to the Convention on Certain Conventional Weapons, the US President noted: “This rule is derived from Article 23 of [the 1907 Hague Regulations] … It thus reiterates a proscription already in place as a matter of customary international law applicable to all weapons.”

United States, Message from the US President to the Senate transmitting Amended Protocol II, Protocol II and Protocol IV to the 1980 Convention on Certain Conventional Weapons, Treaty Doc. 105-1, 7 January 1997.

In a memorandum of law issued in 1997, the Judge Advocate General of the US Department of the Army stated, with reference to Article 23(e) 1907 Hague Regulations: “The law of war prohibits weapons calculated to cause unnecessary suffering.”

United States, Department of the Army, Office of the Judge Advocate General, Memorandum of Law for AMSTA-AR-CCH-C, Picatinny Arsenal, NJ 07806-5000, 25 July 1997, § 4,

In 1998, in a legal review of Oleoresin Capsicum (OC) pepper spray, the Deputy Assistant Judge Advocate General of the US Department of the Navy stated:

The touchstone for legality of a weapon under traditional concepts in the law of war is whether that weapon’s intended use or method of employment is calculated to cause unnecessary suffering …

The Regulations to the Hague Convention on Land Warfare of 1907 codify the prohibition on the employment of arms, projectiles, or material “calculated to cause unnecessary suffering”. This customary prohibition requires a balancing of the military necessity in employing a weapon and the likely suffering occasioned by that employment. Any injury, collateral damage, or general suffering wrought by a weapon’s use should be justified by a military need. Historically, this analysis has involved comparisons to other existing technologies and comparable wounding mechanisms as well as a survey of the practice of other States regarding use of a particular weapon.

…

Oleoresin Capsicum is not calculated (i.e., designed), nor does it in fact cause unnecessary suffering. It is designed specifically to temporarily incapacitate violent or threatening subjects while reducing human suffering and is in consonance with the DoD [Non-Lethal Weapon] program. Its physiological effects, while relatively painful, are temporary and do not rise to the level of unnecessary suffering contemplated in the prohibition … Provided a military necessity justifies its employment, the principle of unnecessary suffering would not preclude employment of OC in appropriate circumstances.

According to the Report on US Practice, it is the opinio juris of the United States that international law forbids weapons or methods of warfare calculated to cause unnecessary suffering or superfluous injury.